Time of flight ranging systems are commonly used in level measurements applications, and referred to as level measurement systems. Level measurement systems are used to determine the distance to a reflective surface (i.e. reflector) by measuring how long after transmission of a burst of energy pulses, an echo is received. Such systems may utilize ultrasonic pulses, pulse radar signals, or microwave energy signals.
Pulse radar and microwave-based level measurement systems are typically preferred in applications where the atmosphere in the container or vessel is subject to large temperature changes, high humidity, dust and other types of conditions which can affect propagation. To provide a sufficient receive response, a high gain antenna is typically used. High gain usually translates into a large antenna size.
Two types of antenna designs are typically found in microwave-based level measurement systems: rod antennas and horn antennas. Rod antennas have a narrow and elongated configuration and are suitable for containers having small opening/flange sizes and sufficient height for accommodating the antenna. Horn antennas, on the other hand, are wider and shorter than rod antennas. Horn antennas are typically used in installations with space limitations, for example, vessels or containers which are shallow.
The level measurement instrument or device comprises a housing and a waveguide (i.e. the antenna). The level measurement instrument is mounted on top of a container or vessel and the antenna extends into the vessel. The level measurement instrument is typically bolted to a flange around the opening of the container. The housing holds the electronic circuitry. The antenna is coupled to the housing and extends into the interior of the vessel. The antenna operates as a transducer and is excited by the circuit to transmit electromagnetic energy pulses into the vessel, and receives the pulses that are reflected by the surface of the material contained in the vessel. To provide a good seal between the antenna and the flange opening O-rings and gaskets are employed. Typically, antenna configurations utilize o-rings or gaskets internally in their assemblies to provide an antenna/vessel mounting configuration that is liquid and/or gas tight.
It will be appreciated that the inclusion of o-rings or gaskets adds to the installation procedure. These components also represent a potential failure point if not periodically inspected and replaced as needed. For o-rings made of rubber there are additional considerations. First, the rubber o-ring becomes brittle at low temperatures and operation at low temperatures can accelerate degradation and subsequent need for replacement. Secondly, the rubber o-ring introduces another material type into the waveguide guide. Rubber as compared to the waveguide is not very conductive of microwave energy waves or pulses and as result can affect the performance of the waveguide.
Accordingly, there remains a need for an antenna configuration for microwave-based level measurement systems which addresses these deficiencies.